Sodium bentonite clay is a non-toxic, natural mineral that has proven to be the most cost-effective pond sealant product for large ponds. You can use sodium bentonite pond sealer in any pond condition, ponds with or without water, with fish, plants, swimming ponds, and ponds that cannot be drained.
Another advantage to having some POOP? Geese Poop especially seals the pond better than bentonite